1. Field of the Invention
The present invention relates to a coating apparatus having an extrusion type coating head and, more particularly, to a coating apparatus with a slit shape of the extrusion type coating head modified so as to enable the formation of a coating in a uniform film thickness on to a support.
2. Description of the Related Art
Hitherto, there has been utilized a variety of methods for applying a coating liquid onto a support, which include a roll coating method, gravure coating method, slide bead coating method, doctor coating method and the like. Recently, an extrusion coating method has attracted attention due to high productivity, good operatability, and high controllability of the thickness of a coat film.
The extrusion coating method is arranged to form a coating in a given film thickness on a support by pressing an edge surface of an extrusion type coating head onto the support at a given amount of tension in a range between each of carrier means such as support rolls and varying a distance between the extrusion type coating head and the support in accordance with an amount of a coating liquid extruded from a slit of the extrusion type coating head through which the coating liquid is applied to the support.
The extrusion type coating head to be employed for extrusion coating may usually comprises a front edge (located at the upstream side with respect to the direction of travel of the support), a back edge (located at the downstream side with respect to the direction of travel of the support), a slit disposed between the front edge and the back edge and a pocket (or a coating liquid reservoir for storing a coating liquid) communicated with the slit. With this arrangement, the coating liquid is first supplied to the pocket from an outside coating liquid supply system and extruded from the slit at a flow rate in proportion to the fluid flow pressure of the coating liquid supplied, thereby applying the coating liquid onto the surface of the support.
For such a conventional extrusion type coating head, its slit usually has its two facing inner slit wall surfaces disposed parallel to each other or it is shaped in a tapered fashion.
A variety of conventional extrusion type coating heads having their tapered-shaped slits have been proposed, for example, in Japanese Patent Unexamined Publications (kokai) Nos. 63-20,069, 63-20,007 and 1-184,071. For each of these conventional tapered-shaped slits, a coating slit consisting of the front edge and the back edge is arranged in such a tapered manner that the opening width of the slit becomes gradually narrower from an inlet for supplying a coating liquid (a boundary portion from the pocket) to an outlet for extruding the coating liquid and applying onto the support. It is disclosed therein that this arrangement can provide a coating of a uniform film thickness by permitting a loss in pressure smaller at the time of extruding the coating liquid from the slit and as a result suppressing a variation in coating film thickness. The prior art as described hereinabove, however, does not disclose or even suggest a structure of the slit in which its coating slit is disposed in such a tapered (inverse-tapered) manner as its width becomes gradually wider from the inlet toward the outlet.
In an actual extrusion coating step for coating with a coating liquid, there may arise the occasion that the coating step is temporarily terminated and it is resumed after a certain period of time. In this case, there is a risk that the coating liquid is caused to coagulate into aggregates due to gelation or for other reasons while remaining in the pocket of the extrusion type coating head during a temporary termination of the coating step for a certain period of time and that the aggregates may be fed in so as accompany the flow of the coating liquid from the pocket through the slit inlet into the slit when the coating is resumed.
As each of the conventional extrusion type coating apparatuses, however, has its slit tapered in such a manner that its width becomes narrower from its inlet end toward its outlet end, the aggregates fed together with the coat-liquid from the inlet into the slit may be caught at the outlet and are difficult to be removed from the outlet. They also may cause coating irregularities or streaks on the surface of the coat and present problems with obtaining a uniform film thickness of the coating.